The constant drive toward small size, lightweight, low cost and low-power sensing systems in all application domains has made high sensitivity and precision microaccelerometers increasingly needed.
These accelerometers are used in numerous applications, such as inertial navigation and guidance, space microgravity measurements, seismology and platform stabilization. Also, as they become manufacturable at low cost with small size, they attain a large potential consumer market in their application as a GPS-aid to obtain position information when the GPS receivers lose their line-of-sight with their satellites.
High precision accelerometers are typically operated closed-loop to satisfy dynamic range, linearity and bandwidth requirements, in addition to high sensitivity and low-noise floor.
Capacitive microaccelerometers are more suitable since they provide stable DC-characteristics and high bandwidth. Such accelerometers may be fabricated by surface micromachining or bulk micromachining. The surface micromachined devices are fabricated on a single silicon wafer. However, they generally have low sensitivity and large noise floor, and thus cannot satisfy the requirements of many precision applications.
Some high resolution accelerometers are bulk micromachined and use multiple wafer bonding as part of their manufacturing process. This wafer bonding is a complex fabrication step, and hence results in lower yield and higher cost. Also, forming damping holes in the thick bonded wafers is difficult, and thus special packaging at a specified ambient pressure is typically needed to control the device damping factor. Finally, due to wafer bonding, these devices show higher temperature sensitivity and larger drift especially if glass wafers are used.
The above-noted patent application entitled "Microelectromechanical Capacitive Accelerometer And Method Of Making Same" utilizes a single wafer fabrication technology with damping holes. However, fabrication of the accelerometer requires double side processing and lead transfer from both sides of the wafer. As shown in FIG. 1, the accelerometer, generally indicated at 10, includes a proof mass 12 suspended by compliant beams 14 between two fixed and rigid electrodes 16. In the presence of an external acceleration, the proof mass 12 moves from its center position and thus C.sub.S1 and C.sub.S2 change in opposite directions. The proof mass 12 is rebalanced to its center position by applying an electrostatic force to either the top electrode 16 or the bottom electrode 16.
U.S. Pat. No. 5,345,824 discusses a monolithic capacitive accelerometer with its signal conditioning circuit fabricated using polysilicon proof mass and surface micromachining.
U.S. Pat. No. 5,404,749 discusses a boron-doped silicon accelerometer sensing element suspended between two conductive layers deposited on two supporting dielectric layers.
U.S. Pat. No. 5,445,006 discusses a self-testable microaccelerometer with a capacitive element for applying a test signal and piezoresistive sense elements.
U.S. Pat. No. 5,461,917 discusses a silicon accelerometer made of three silicon plates.
U.S. Pat. No. 5,503,285 discusses a method for forming an electrostatically force rebalanced capacitive silicon accelerometer. The method uses oxygen implantation of the proof mass to form a buried oxide layer and bonding of two complementary proof mass layers together. The implanted oxide layer is removed after bonding to form an air gap and release the proof mass.
U.S. Pat. No. 5,535,626 discusses a capacitive microsensor formed of three silicon layers bonded together. There is glass layer used between each two bonded silicon pairs.
U.S. Pat. No. 5,540,095 discusses a monolithic capacitive accelerometer integrated with its signal conditioning circuitry. The sensor comprises two differential sense capacitors.
U.S. Pat. No. 5,559,290 discusses a capacitive accelerometer formed of three silicon plates, attached together using a thermal oxide interface.
U.S. Pat. No. 5,563,343 discusses a lateral accelerometer fabricated of a single crystal silicon wafer.
U.S. Pat. No. 5,605,598 discloses a monolithic micromechanical vibrating beam accelerometer having a trimmable resonant frequency and method of making same.
U.S. Pat. Nos. 5,594,171 and 5,830,777 disclose capacitance-type acceleration sensors and methods for manufacturing the sensors. The sensors include a mass portion having a plurality of movable electrodes. The sensors also include a plurality of stationary electrodes. The sensors are manufactured on a single-side of a substrate.
U.S. Pat. No. 5,665,915 discloses a semiconductor capacitive acceleration sensor. The construction of the sensor includes a base substrate having a first electrode attached to the top of the substrate. The sensor also includes a second electrode positioned between the substrate and the first electrode. The first electrode is a stationary electrode and the second electrode is a movable electrode.
U.S. Pat. No. 5,719,336 discloses a capacitive acceleration sensor having a first fixed electrode, a second fixed electrode, a first movable electrode, and a second movable electrode. The stationary electrodes are positioned in a configuration surrounding the movable electrodes.
U.S. Pat. Nos. 5,392,651; 5,427,975; 5,561,248; 5,616,844; and 5,719,069 disclose various configurations of microminiature accelerometers having both stationary and movable electrodes, wherein the electrodes are arranged in various configurations.
The paper entitled "Advanced Micromachined Condenser Hydrophone" by J. Bernstein et al, Solid-State Sensor and Actuator Workshop, Hilton Head, South Carolina, June, 1994, discloses a small micromechanical hydrophone having capacitor detection. The hydrophone includes a fluid-filled variable capacitor fabricated on a monolithic silicon chip.
The paper entitled "Low-Noise MEMS Vibration Sensor for Geophysical Applications" by J. Bernstein et al., DIGEST OF HILTON-HEAD SOLID STATE SENSOR AND ACTUATOR WORKSHOP, pp. 55-58, June, 1998, discloses an accelerometer having trenches etched in its proof mass to reduce damping and noise floor.
The paper entitled "High Density Vertical Comb Array Microactuators Fabricated Using a Novel Bulk/Poly-Silicon Trench Refill Technology", by A. Selvakumar et al., Hilton Head, S.C., June 1994, discloses a fabrication technology which combines bulk and surface micromachining techniques. Trenches are etched and then completely refilled.
Numerous U.S. patents disclose electroplated microsensors such as U.S. Pat. Nos. 5,216,490; 5,595,940; 5,573,679; and 4,598,585.
Numerous U.S. patents disclose accelerometers such as U.S. Pat. Nos. 4,483,194 and 4,922,756.
U.S. Pat. No. 5,146,435 discloses an acoustic transducer including a perforated plate, a movable capacitor plate and a spring mechanism, all of which form a uniform monolithic structure from a silicon wafer.